The cellular and subcellular distribution of mercaptoundecahydro-closo-dodecaborate (BSH) were investigated in glioblastoma multiforme tissue sections of seven patients, whom having received BSH prior to surgery. To achieve this, several microscopic techniques were used. Direct detection of boron was performed by x-ray photoelectron emission spectromicroscopy (X-PEEM), electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI). For visualization of BSH in light-, fluorescence microscopy and transmission electron microscopy (TEM) antibodies against BSH were used.These microscopic techniques show that BSH is associated with the cell membrane as well as with the chromatin in the nucleus. With use of specific antibodies against different tumor specific epi-topes, BSH could be found predominantly (approx. 90 %) in the cytoplasm of GFAP-positive cells of all but two patients. The latter were significantly younger (33 and 38 years versus to 46-71 (mean 60) years). Based on the expression of EGFR and p53, the tumors of five patients could be characterized as primary or secondary glioblastoma. Although the tissue regions of EGFR and p53 overexpression do not seem to correspond with the regions of high BSH incidence, muta-tions during tumor development could be the reason for the distinct accumulation of BSH. No direct correlation between BSH uptake and expression of CD44, vWF, laminin and Ki-67 has been observed.By x-ray photoelectron emission spectromicroscopy boron was found in a reduced chemical state (with respect to boron in BSH). The reduced boron species was found in tissue in the same general areas, which stained positive for BSH in light microscopy. The present work presents for the first time evidence that a proportion of the BSH injected into the patient could have been bound or metabolized in vivo.